1. Field of the Invention
The present invention relates to a monitoring apparatus for monitoring the transmission state in an optical transmission path through which light containing one or more light signals having different wavelengths propagates, a monitoring method, an optical amplification system including the monitoring apparatus, a method of controlling the optical amplification system, and an optical transmission system.
2. Related Background Art
An optical transmission system using a WDM (Wavelength Division Multiplexing) communication scheme includes an optical fiber transmission network through which WDM signals containing one or more light signals having different wavelengths propagates, and can perform large-capacity, high-speed communication. This optical transmission system includes an optical amplifier for amplifying WDM signals altogether, an optical ADM (Add-Drop Multiplexer) for dropping and adding some light signals contained in the WDM signals, and the like as well as an optical fiber transmission path as a transmission medium for light signals.
In the optical transmission system having the above structure, monitoring control of the optical amplifier is one of the important subjects. More specifically, the optical power of each amplified light (light signals) optically amplified by the optical amplifier is required to be controlled to a constant level in either of the following cases: a case wherein the number of light signals sent from the transmitter varies, a case wherein the number of light signals propagating a long an optical transmission path varies due to dropping/adding of light signals by the optical ADMs placed midway along the optical transmission path, and a case wherein a transmission loss in the optical transmission path or the like varies. In order to solve this problem, various proposals have been made.
For example, in xe2x80x9cControl of Optical Output Level for WDM Optical Fiber Amplifierxe2x80x9d (first prior art) proposed in the 1996 IEICE communication society conference B-1096, an acoustooptic filter whose passing wavelength characteristics change in accordance with a change in the frequency of ultrasonic waves is used as an optical transmission wavelength selection element. The acoustooptic filter is controlled by a sweep circuit, sweeps a band of 1,545 nm to 1,557 nm in a cycle of 400 xcexcs, and converts WDM signals into a pulse string on the time axis. This pulse string is photoelectrically converted. A wave counter then detects the number of light signals contained in the WDM signals. Automatic level control (ALC) of the optical amplifier is performed on the basis of the information about the detected number of light signals. Note that the above acoustooptic filter is disclosed in xe2x80x9cWide Tunable Range and Low Sidelobe Level of Double-stage Polarization Independent Acousto-optic Tunable Filterxe2x80x9d proposed in the 1996 IEICE general conference C-254.
According to the method proposed in xe2x80x9cCharacteristics of EDFA with Automatic Level Control for change of number of the wavelengths and input Signal levelxe2x80x9d proposed in the 1996 IEICE communication society conference B-1092 (second prior art), pilot light in the amplification band of an optical amplifier is dropped from an optical transmission path by a branching element placed on the output side of the optical amplifier, the optical power of the dropped pilot light is detected, and the optical amplifier is controlled to keep the optical power of this pilot light constant. As a technique similar to the second prior art, for example, the technique disclosed in Seo Yeon Park and Sang-Yung Shin, xe2x80x9cGain and power controlled EDFA and WDM optical networksxe2x80x9d, 2nd Optoelectronics and Communications Conference (OECC""97) Technical Digest, July 1997, Seoul KOREA is known.
In addition, according to Takashi Ono, xe2x80x9cFiber grating wavelength monitor for optical amplifier control and administration in WDM transmission systemsxe2x80x9d, First Optoelectronics and Communications Conference (OECC""96) Technical Digest, July 1996, Makuhari Messe 17B3-2 (third prior art), part of WDM signals (some light signals) is dropped by a branching element connected to the output terminal of an optical amplifier and output as short pulses by an acoustooptic switch. Delays corresponding to the wavelengths of the respective light signals of the short pulses are given to the respective light signals through an optical circulator and optical fiber grating. The light signals having different wavelengths, to which the delays are given in this manner, are converted into pulses arranged along the time axis. The number of light signals contained in the WDM signals, operating wavelength band, the powers of the respective light signals are obtained from the number, positions, and optical powers of these pulses. In the third prior art, automatic level control (ALC) of the optical amplifier is performed on the basis of the obtained number of light signals contained in the WDM signals.
The present inventors found the following problems upon examination of the first and third prior arts. The first to third prior arts use special optical elements such as acoustooptic filters, acoustooptic switches, and optical circulators, and the second prior art uses pilot light. For this reason, these conventional systems need to have complicated arrangements, and become expensive systems.
In addition, since the first and third prior arts use acoustooptic filters and acoustooptic switches, variations in the power of a light signal of WDM signals which have specific wavelengths cannot be detected, although the number of light signals contained in the WDM signals can be detected. The second prior art using pilot light can neither detect the number of light signals contained in WDM signals nor detect variations in the power of light signals of the WDM signals which has a specific wavelength. The first to third prior arts cannot therefore discriminate whether a cause of a variation in the optical power of WDM signals is a variation in the optical power of a light signal having a specific wavelength or a loss variation in an optical transmission path. This problem is serious especially in an optical transmission system using an optical ADM.
The present invention has been made to solve the above problems, and has as its object to provide an optical transmission monitoring apparatus having a simple structure capable of identifying a cause of variations in the optical power of WDM signals, a monitoring method, an optical amplification system including the optical transmission monitoring apparatus, a method of controlling the optical amplification system, and an optical transmission system.
An optical transmission monitoring apparatus according to the present invention is an apparatus for monitoring a transmission state in an optical transmission path through which light belongs in a signal wavelength band and containing one or more light signals having different wavelengths propagates. This apparatus uses light in the signal wavelength band or light in a wavelength band different from the signal wavelength band as monitoring light signals.
More specifically, the optical transmission monitoring apparatus according to the present invention comprises first and second photodetectors and a monitoring section for monitoring a transmission state in an optical transmission path by using the detection results obtained by the first and second photodetectors.
The first photodetector detects at least one of the optical power of one or more monitoring light signals of light in the first wavelength band, which is included in a monitor wavelength band in which light propagating through an optical transmission path and containing one or more monitoring light signals having different wavelengths belongs, and the optical power of noise of the light in the first wavelength band. The second photodetector detects at least one of the optical power of one or more monitoring light signals of light in the second wavelength band included in a monitor wavelength band, and the optical power of noise of the light in the second wavelength band.
The above monitor wavelength band may be a wavelength band which differs from the signal wavelength band and partly overlaps the signal wavelength band. When the monitor wavelength band differs from the signal wavelength band, light propagating in an optical transmission path and belonging in the monitor wavelength band is dropped from the optical transmission path by a demultiplexer. The dropped light in the monitor wavelength band is guided to the optical transmission monitoring apparatus to be used for optical transmission monitoring. When the monitor wavelength band overlaps the signal wavelength band, light signals in the overlapping wavelength band can be used as monitoring light. By placing at least one demultiplexer in an optical transmission path, therefore, light belonging in the signal wavelength band and propagating through the optical transmission path is partly dropped. The dropped light is guided to the optical transmission monitoring apparatus to be used for optical transmission monitoring. In this manner, at least some light signals of the light signals dropped from the optical transmission path can be used as monitoring light signals.
In the above monitor wavelength band, the first and second wavelength bands may be different wavelengths. The first and second wavelength bands may partly overlap each other on condition that they do not coincide with each other. In the mode in which the first and second wavelength band overlap each other, the bandwidth of the first wavelength band is greater than that of the second wavelength band, and the second wavelength band is preferably included in the first wavelength band.
The above monitoring section also compares the variation amount of optical power per unit time in the first wavelength band which is detected by the first photodetector with the variation amount of optical power per unit time in the second wavelength band which is detected by the second photodetector. The monitoring section then identifies a cause of a optical power variation in the optical transmission path as a monitoring target on the basis of the obtained comparison result. More specifically, when the optical power variations in the first wavelength band exhibit a tendency similar to that of the optical power variations in the second wavelength band, transmission loss variations in the optical transmission path (when an optical amplifier is placed in the optical transmission path, gain variations in the optical amplifier are included) are determined. If the optical power variations in the first wavelength band exhibit a tendency different from that of the optical power variations in the second wavelength band, it is determined that the number of light signals propagating through the optical transmission path has increased/decreased. Since this monitoring section uses variations in optical power in both the first and second wavelength bands, light to be detected by the first and second photodetectors may be monitoring light in the monitor wavelength band (corresponding to light signals when the signal wavelength band coincides with the monitor wavelength band), noise such as amplified spontaneous emission (ASE), or light containing these light signals. According to another mode, the light signals to be detected by the first photodetector need not be identical to those detected by the second photodetector. Therefore, for example, the optical power of light signals (monitoring light signals) may be detected by the first photodetector, and the optical power of noise such as ASE excluding the light signals may be detected by the second photodetector. Variations in the optical powers of the detected light signals may be used to monitor the optical transmission path.
A change in the number of light signals can be monitored by the above arrangement when light signals are used as monitoring light signals, and the number of light signals as a group of monitoring light signals increases/decreases in either of the first and second wavelength bands in the monitor wavelength band. More specifically, when the numbers of light signals increase/decrease at the same ratio in the first and second wavelength bands in the monitor wavelength band, since optical powers vary in the same manner in both the wavelength bands, loss variations in the optical transmission path are erroneously detected. In an optical transmission monitoring apparatus designed to solve such a problem, light signals in a wavelength band other than the signal wavelength band are made to propagate as some monitoring light signals, and the optical powers of at least three types of light (containing different light signals) in the monitor wavelength band are detected. This makes it possible to monitor the transmission state in which the number of light signals has changed in the above manner.
In the above mode, light in the signal wavelength band corresponds to light in the first wavelength band of the monitor wavelength band, and light propagating through the optical transmission path independently of the light signal corresponds to light in the second wavelength band. The optical transmission monitoring apparatus further comprises a third detector for detecting at least the optical power of light containing noise in the first wavelength band. With this arrangement, the optical transmission monitoring apparatus detects the optical power of first light (may contain noise in the first wavelength band or monitoring light in the second wavelength band) containing at least monitoring light signals (light signals) in the first wavelength band by using the first detector, detects the optical power of second light containing at least monitoring light in the second wavelength band by using the second detector, and detects the optical power of third light containing at least noise in the first wavelength band by using the third detector. The monitoring section compares the variation amount of optical power of the first light per unit time which is obtained by the first detector with the variation amount of optical power of the second light per unit time which is obtained by the second detector, and the variation amount of optical power of the third light per unit time, obtained by the third detector, and identifies a cause of optical power variations in the optical transmission path on the basis of the comparison result.
An optical transmission monitoring apparatus according to the present invention may have an arrangement for monitoring an optical transmission state by using a light signal of 1-channel in the signal wavelength band as pilot light. In this case, light in the signal wavelength band corresponds to light in the monitor wavelength band, and a wavelength band in which pilot light of 1-channel belongs and which is included in the signal wavelength band corresponds to the second wavelength band. According to this arrangement, therefore, a monitor wavelength band is set such that the second wavelength band is included in the first wavelength band. The monitoring section compares the variation amount of optical power of light signals and noise as a monitoring light signal in the first wavelength band per unit time with the variation amount of optical power of pilot light as monitoring light in the second wavelength band per unit time, and identifies a cause of variations in optical power in the optical transmission path on the basis of the comparison result. Preferably, the optical transmission monitoring apparatus may further comprise a first system for detecting an abnormality in transmission of at least a light signal of 1-channel selected from light signals in a wavelength band of the second wavelength band which overlaps the first wavelength band, and a second system for reselecting light signals to be selected as monitoring light in the second wavelength band from light signals in a wavelength band of the signal wavelength band which over laps the first wavelength band. This is -because a channel as a monitoring target may fail due to some reason or its use may be stopped.
When an optical amplifier is placed in an optical transmission path, the optical transmission monitoring apparatus according to the present invention can also be used as a control apparatus for the optical amplifier. More specifically, the optical transmission monitoring apparatus is placed at a predetermined position on the optical transmission path which the amplified light outputted from the optical amplifier reaches. When the amplification characteristics of the optical amplifier depend on wavelength, the center wavelength of the first wavelength band and that of the second wavelength band are preferably set to coincide with a wavelength at which the gain becomes almost equal to the average gain of the optical amplifier in the signal wavelength band in order to minimize the influences of the wavelength dependency of the optical amplifier. In addition, the bandwidths of the first and second wavelength bands are preferably set on the basis of at least the difference between the amplification wavelength band of the optical amplifier and the monitor wavelength band or between the number of monitoring light signals belonging in the monitor wavelength band.
An optical transmission monitoring method according to the present invention is implemented by using the above optical transmission monitoring apparatus. More specifically, according to the monitoring method, the variation amount of at least one of the optical power of one or more monitoring light signals of light in a first wavelength band included in a monitor wavelength band, in which light propagating through the optical transmission path and containing one or more monitoring light signals having different wavelengths belongs, and the optical power of noise of the light in the first wavelength band is detected per unit time, and the variation amount of at least one of the optical power of one or more monitoring light signals of light in a second wavelength band included in the monitor wavelength band and the optical power of noise of the light in the second wavelength band is detected. In the monitoring method, the detected variation amount of optical power per unit time in the first wavelength band is compared with the detected variation amount of optical power per unit time in the second wavelength band, and a cause of variations in optical power in the optical transmission path is identified on the basis of the comparison result.
An optical amplification system according to the present invention comprises at least an optical amplifier for amplifying altogether light belonging in a signal wavelength band and propagating through an optical transmission path and an optical transmission monitoring apparatus placed at a predetermined position on the optical transmission path which amplified light outputted from the optical amplifier reaches. This optical transmission monitoring apparatus has the above structure. The optical amplification system may further comprise a variable attenuator which is placed at least at a position, on the optical transmission path, through which light signals in the signal wavelength band pass before reaching the optical transmission monitoring apparatus or a position through which light signals pass after passing through the optical transmission monitoring apparatus and attenuates the light signals by a predetermined amount. In this case, the monitoring section of the optical transmission monitoring apparatus adjusts the attenuation amount of the variable attenuator in accordance with the transmission state in the optical transmission path.
In a method of controlling an optical amplification system having such a structure as well, as described above, the variation amount of at least one of the optical power of one or more monitoring light signals of light in a first wavelength band included in a monitor wavelength band, in which light propagating through the optical transmission path and containing one or more monitoring light signals having different wavelengths belongs, and the optical power of noise of the light in the first wavelength band is detected per unit time, and the variation amount of at least one of the optical power of one or more monitoring light signals of light in a second wavelength band included in the monitor wavelength band and the optical power of noise of the light in the second wavelength band is detected per unit time. The detected variation amount of optical power per unit time in the first wavelength band is compared with the detected variation amount of optical power per unit time in the second wavelength band, and an increase/decrease in the number of light signals is determined on the basis of the comparison result. At least adjustment of the gain of the optical amplifier or adjustment of the variable attenuator is performed in accordance with the increase/decrease in the number of light signals on the basis of the determination result.
An optical transmission system according to the present invention comprises at least one or more repeater stations placed at predetermined positions on an optical transmission path through which light belonging in a signal wavelength band and containing one or more light signals having different wavelengths propagates. Each repeater station includes a section monitoring apparatus having the same structure as that of the above optical transmission monitoring apparatus.
Each repeater station may further comprise an optical amplifier for amplifying altogether light belonging in the signal wavelength band and propagating through the optical transmission path. In this arrangement, the monitoring section of the optical transmission monitoring apparatus adjusts the gain of the optical amplifier in accordance with the transmission state in the optical transmission path.
Each repeater station may further comprise a variable attenuator for attenuating light signals in the signal wavelength band by a predetermined amount. In this arrangement, the monitoring section of the optical transmission monitoring apparatus adjusts the variable attenuator in accordance with the transmission state in the optical transmission path. Each repeater station may comprises an optical input/output element such as an optical ADM for dropping some light signals from the light signals propagating through the optical transmission path and adding new light signals. In this optical transmission system, in particular, either of the first and second wavelength bands may include some of light signals to be dropped or added by the optical input element. This arrangement is preferable because no monitoring light signals need to be prepared independently of light signals.
According to this optical transmission system, each repeater station having the above structure may further comprise a system control section for monitoring optical power variations in the overall optical transmission system on the basis of the variation amount of optical power of some light signals separated from the light belonging in the monitor wavelength band and reaching the repeater station per unit time and the variation amount of optical power of some light signals separated from the light belonging in the monitor wavelength band and outputted from the repeater station per unit time. In this arrangement, this system control section corrects control performed by the monitoring section of the optical transmission monitoring apparatus in each repeater station on the basis of the variation amount of optical power of each of input/output light signals in each repeater station.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given byway of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.